Anthrax—primarily a disease of herbivorous animals but of rare occurrence in humans—is caused by Bacillus anthracis. Cutaneous anthrax is acquired via injured skin or membranes, entry sites where the spore germinate into vegetative cells. Proliferation of vegetative cells results in gelatinous edema. Alternatively, inhalation of the spores results in high fever and chest pain. Both types can be fatal unless the invasive aspect of the infection can be intercepted. Bacillus anthracis is a biological warfare (BW) agent. Ten grams of anthrax spore can kill as many people as a ton of the chemical warfare agent, sarin. Due to the highly lethal nature of anthrax and BW agents in general, there is great need for the development of sensitive and rapid BW agent detection. Current detection technology for biological warfare agents have traditionally relied on time-consuming laboratory analysis or onset of illness among people exposed to the BW agent.
In theory, the use of specific antibodies or distinguishing DNA probes are the two approaches to modernizing detection technology in this field. However, antibody-based detection of threat agents suffers from drawbacks. For example, interference from other environmental contaminants precludes detection, or detection limits of current levels fail to meet the detection thresholds set by governmental testing protocols. Alternatively, the threat agent, such as with anthrax spore, may be poorly immunogenic.
Since a sample suspected of containing a BW agent like B. anthracis could contain such a small yet lethal amount of spores, and an overwhelming amount of other interfering materials, the ability to amplify the agent's genomic material affords a choice of target sites for developing signature probes for specific detection of that agent. Development of highly discriminating techniques are crucial to achieving the stated goals of rapid and sensitive BW detection.
Current PCR-based detection methods of B. anthracis rely on the use of primers amplifying tripartite exotoxin genes and/or the polyglutamic capsule genes (Jackson et al, Proc. Natl. Acad. Sci, 95: 1224-9 (1998)). Both sets of genes comprise virulence factors and are located on the two indigenous plasmids of anthrax bacteria, pXO1 (174 kbp; toxin) and pXO2 (95 kbp; capsule). Under normal conditions, the two plasmids in B. anthracis do not move across the related bacilli of the “B. cereus group” (which is comprised of B. anthracis, B. cereus, B. thuringiensis and B. mycoides (although B. mycoides does not produce toxin and therefore may be grouped differently from the other to three members)). However, under certain conditions, these plasmids are known to be transferred from B. anthracis to B. cereus and B. thuringiensis (Ruhfel et al, J. Bact., 157: 708-11 (1984)). Yet B. cereus and B. thuringiensis containing one or both of these plasmids do not cause anthrax. Therefore, detection of anthrax based solely on virulence factors can give rise to a false-positive determination.
Two chromosomal DNA fragment sequences from B. anthracis have been previously identified and used in identifying the presence of B. anthracis bacteria. One, designated Ba813, is a 277 bp long DNA fragment (Patra et al., FEMS Microbiol. 15: 223-231 (1996)) and the other, vrrA, is a region of sequence variability containing variable repeats (caa tat caa caa) (Anderson et al., J. Bacteriol. 178: 377-384 (1996)).
Additionally, Yamada et al (U.S. Pat. No. 6,087,104) identified unique regions of the DNA gyrase sub-unit B (gyrB) gene for each of the closely related bacteria of the B. cereus group, and designed oligonucleotide primers corresponding to those unique regions for amplification-based detection methods. However, amplification of DNA segments unique to each of the B. cereus group members occurred only when the correct target strain DNA by itself was present in the PCR protocol.
However, since the development of more rapid and more sensitive BW detection methodologies is of such importance to the military as well as public health sectors of the U.S. government, there is great need to continue the process of identifying, cloning, and sequencing of polymorphic DNA markers from chromosomal DNA of threat agents. With this purpose in mind, comprehensive libraries of BW agent-specific signature sequences can be built, and from there useful diagnostic primers and probes can be designed for highly discriminating detection methods. The present invention as herein described fulfills these objectives.